This invention relates to improved catalysts for the dehydrogenation of hydrocarbons to corresponding more-unsaturated hydrocarbons, more particularly, to the production of vinyl aromatic hydrocarbons from alkyl aromatic hydrocarbons and to the production of olefins from the corresponding more-saturated aliphatic hydrocarbons.
The vinyl benzenes and butadienes play a particularly important role in the preparation of synthetic rubbers, plastics and resins. The polymerization of styrene, for example, with various comonomers such as butadiene to produce synthetic rubbers is well known as is the polymerization of styrene to produce polystyrene resins.
Styrene and butadiene are typically produced from ethylbenzene and butylene, respectively, by dehydrogenation over solid catalysts in the presence of steam and at temperatures ranging from 500.degree. to 800.degree. C. The class of catalysts found to be the most effective for this process contains a predominant amount of iron oxide, promoted by potassium carbonate, and stabilized by chromium oxide.
The activity of a catalyst is measured by the amount of starting material converted at a given temperature to another material and the selectivity is the percent of the converted material which is the desired product. Any improvement which results in either increasing the selectivity or the activity without lowering the other is economically attractive since the result is that the yield of the product (moles of desired product produced per mole of reactant) has been increased. An increase of only a few tenths of a percent in the selectivity can result in a substantial savings of starting materials while an increase in activity can substantially reduce capital expenditure and energy consumption.
The prior art catalysts used in the dehydrogenation of alkyl aromatics to alkenyl aromatics, for example, ethylbezzene to styrene as previously discussed, are widely known. Early dehydrogenation patents describe the use of cerium oxide as the major active ingredient. Thus, for example, U.S. Pat. No. 1,985,844 discloses the use of cerium oxide precipitated on broken pieces of clay to dehydrogenate ethylbenzene below atmospheric pressure. Another, U.S. Pat. No. 2,036,410, discloses the use of cerium oxide promoted with oxides of tungsten, uranium, and molybdenum.
Various catalysts, dehydrogenation conditions and other operating data are disclosed by Pitzer in U.S. Pat. No. 2,866,790 relating to the use of a catalyst composition including potassium carbonate, chromium oxide, and iron oxide. Other catalysts and procedures are also shown by Gutzeit, U.S. Pat. No. 2,408,140: Eggertsen, et al., U.S. Pat. No. 2,414,585; Hills, et al, U.S. Pat. No. 3,360,579; and Riesser, U.S. Pat. No. 4,098,723.
O'Hara, in U.S. Pat. No. 3,904,552, discloses the use of cerium and molybdenum in alkali promoted iron oxide dehydrogenation catalysts.
MacFarlane, in U.S. Pat. No. 3,223,743, teaches that it is very difficult to obtain a catalyst which has both high selectivity and high activity since as one increases, the other usually decreases. To overcome this he teaches the use of two layers of catalyst, the first layer with high selectivity (lower activity) and a second layer with high activity (lower selectivity). The first layer may contain 20-80% by weight iron and 0.5-5% by weight cerium oxide.
Riesser, in U.S. Pat. No. 4,144,197, discloses the use of vanadium to improve the selectivity of a potassium promoted dehydrogenation catalyst containing 20-95% by weight ferric oxide and 0.01-50% by weight cerous oxide. Data in the '197 patent is consistent with the teaching of MacFarlane with respect to vanadium addition since increasing amounts increase the selectivity at the sacrifice of activity. Also disclosing the use of cerium as a promoter in iron oxide based catalysts is U.S. Pat. No. 4,467,046.
A copending application of two of the inventors of the present invention discloses a catalyst formulation containing low concentrations of iron in combination with high concentrations of potassium and cerium. The referenced application, Ser. No. 69,464, filed July 1, 1987 is entitled "Dehydrogenation Catalyst".
Patents which show the use of copper as a stablizer for dehydrogenation catalysts include U.S. Pat. Nos. 2,395,875 and 2,395,876 which disclose catalysts containing a major amount of magnesium oxide, a minor amount of iron oxide together with an alkali or alkaline earth promoter and a copper oxide stablizer. A zinc oxide based catalyst containing a minor amount of iron oxide, promoted with potassium oxide and stabilized with copper oxide is disclosed in U.S. Pat. No. 2,418,888 while U.S. Pat. No. 2,418,889 discloses a similar beryllium oxide based catalyst. U.S. Pat. No. 2,426,829 which discloses an alkali promoted iron oxide dehydrogenation catalyst indicates that oxides of aluminum, zinc or copper may be added as a stabilizers.
Other patents teaching the use of copper oxide as a stabilizer for alkali-promoted iron oxide catalysts are U.S. Pat. Nos. 3,387,053; 3,448,058; 3,542,897; 4,064,187 and 4,134,858. Other catalysts in which copper is the primary or major component include those disclosed in U.S. Pat. Nos. 4,279,777; 4,334,116 and 4,590,324.
It has now been discovered that an alkali promoted iron/cerium dehydrogenation catalyst can be improved with respect to its activity, with little or no effect on selectivity, by the addition of a copper compound.
While copper has been employed as a stabilizer in some iron dehydrogenation catalysts, as above indicated, its combination with cerium in the presently disclosed catalyst compositions is novel and provides unexpected benefits.